Modern high bypass turbofan engines incorporate wide chord fan blades for improved aerodynamic efficiency. Hollow fan blade airfoils may be used to minimize system weight. Hollow airfoils may be manufactured by diffusion welding premachined airfoil sections together. A common internal core construction relies on the use of radial (or span-wise) ribs for centrifugal loading. These ribs support the airfoil skins against pressure differentials, bird impact, and running distortion. Radial ribs also provide stiffness for frequency tuning. That is, by adjusting the thickness, length and/or number of ribs, the resonant frequencies of the blade may be adjusted to acceptable values. Other characteristics of the radial ribs may also be adjusted to achieve desired airfoil characteristics.
Other designs also incorporate ribs aligned along the chord of the airfoil, substantially perpendicular to the radial ribs. This grid configuration is sometimes referred to as a "waffle" core. These ribs, while providing improved skin support and chord-wise stiffness, have their own drawbacks.
First, the diffusion bonds of the chord-wise ribs are aligned in a direction sensitive to centrifugal and flexural mode vibratory stresses. Diffusion bonds may exhibit local flaws or discontinuities (e.g. mismatched surfaces) which behave as cracks or stress concentrations. These defects effectively reduce the fatigue strength of the airfoil, and hence the life of the airfoil.
The chord-wise ribs also add significant weight to the airfoil itself. While chord-wise ribs provide skin support, much of the rib mass near the airfoil mean camber line supplies little or no additional stiffness. The additional "dead" or "parasitic" weight introduced by these ribs also lowers the flexural resonant frequencies of the airfoil.
Finally, it may be necessary to introduce gas to re-inflate the "waffle" chambers after the two halves of the airfoil are diffusion bonded. The ribs of the "waffel" chamber, therefore, may incorporate access channels which can introduce stress concentrations beyond those introduced by the diffusion bond alone. In other words, by introducing discontinuities (e.g., step changes) in the rib, the access channels introduce changes in the stress concentrations at the edges of the access channels.
Chord-wise stiffeners have also been used on the outer surfaces of solid airfoils to provide increased resistance to bird impact. However, external stiffeners may degrade the aerodynamic properties of the airfoil.